Muscular dystrophy, which is a genetic muscular disease, is roughly classified into Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD). DMD is the most frequently occurring genetic muscular disease and occurs at a ratio of 1 per 3,500 male births. DMD patients show symptoms of weakening of muscles in their childhood; thereafter, muscular atrophy progresses consistently and results in death at the age of around 20. Currently, there is no effective therapeutic for DMD. Development of therapeutics is strongly demanded by DMD patients throughout the world. BMD in many cases occurs in adulthood and most of the patients are capable of normal survival though slight weakening of muscles is observed. Mutations of deletions in the dystrophin gene have been identified in ⅔ of DMD and BMD cases. The progress of clinical symptoms in DMD or BMD patients is predictable depending on whether such deletions disrupt the translational reading frame of mRNA or maintain that reading frame (Monaco A. P. et al., Genomics 1988: 2:90-95). Although molecular biological understanding of DMD has been thus deepened, no effective method for treating DMD has been established yet.
When DMD patients have a frame shift mutation, dystrophin protein disappears completely from patients' skeletal muscles. On the other hand, dystrophin protein is produced from in-frame mRNA in BMD patient-derived muscle tissues, though the protein is incomplete. As a method for treating DMD, there is known a method in which an out-frame mutation (the reading frame of amino acids is shifted) is converted to an in-frame mutation (the reading frame is maintained) by modifying dystrophin mRNA (Matsuo M., Brain Dev 1996; 18:167-172). Recently, it has been reported that the mdx mouse synthesized a deletion-containing dystrophin as a result of induction of exon skipping with an oligonucleotide complementary to the splicing consensus sequence of the dystrophin gene (Wilton S. D. et al., Neuromusc Disord 1999: 9:330-338; Mann C. J. et al., Proc Natl Acad Sci USA 2001: 98:42-47). In these studies, exon skipping is induced using as a target the splicing consensus sequence located on the border between two exons.
It is asserted that splicing is regulated by splicing enhancer sequences (SESs). In fact, it has been demonstrated that by disrupting the SES within exon 19 of the dystrophin gene with an antisense oligonucleotide complementary thereto, complete skipping of exon 19 occurs in normal lymphoblastoid cells (Takeshima Y. et al., J Clin Invest 1995: 95:515-520; Pramono Z. A. et al., Biochem Biophys Res Commun 1996: 226:445-449).
It has been also reported that by introducing an oligonucleotide complementary to the SES within exon 19 of the dystrophin gene to thereby induce exon skipping, a deletion-containing dystrophin was successfully produced in muscular cells derived from DMD patients carrying exon 20 deletion (Takeshima Y et al., Brain & Development 2001: 23:788-790; Japanese Unexamined Patent Publication No. H11-140930; Japanese Unexamined Patent Publication No. 2002-10790). This indicates that repairing of the reading frame shift by inducing exon 19 skipping with an antisense oligonucleotide complementary to the SES within exon 19 of the dystrophin gene results in production of a dystrophin protein whose function is partially restored; and thus it is possible to change DMD to BMD. If it is possible to convert DMD, a severe myoatrophy, to slight BMD, prolonging patients' lives can be expected.
At present, oligonucleotide analogues having stable and excellent antisense activity are being developed (Japanese Unexamined Patent Publication No. 2000-297097).
It is an object of the present invention to provide therapeutics with broader applicable range and higher efficacy, by improving antisense oligonucleotides to the SES within exon 19, 41, 45, 46, 44, 50, 55, 51 or 53 of the dystrophin gene.